1. Field of the Invention
The present invention relates to an insulating DC-DC converter including a synchronous rectifier.
2. Description of the Related Art
An example of a DC-DC converter is shown in the schematic circuit diagram in FIG. 6, and examples of schematic operating waveforms are shown in FIG. 7. Here, a forward converter is described as an example. This circuit is disclosed in Japanese Unexamined Patent Application Publication No. 2000-262051.
The DC-DC converter in FIG. 6 includes a main transformer 2, a power switch 3 (N-channel metal-oxide semiconductor field-effect transistor (MOSFET)), a control IC 4, a rectifying synchronous rectifier 5 (N-channel MOSFET), a commutating synchronous rectifier 6 (N-channel MOSFET), a choke coil 7, a capacitor 8, and an early turnoff circuit 16.
The main transformer 2 includes a primary coil 2A, a secondary coil 2B, and an auxiliary coil 2C. The control IC 4 has a power switch driving circuit (not shown) therein, an output terminal OUT for outputting a power switch driving signal generated in the power switch driving switch, and a ground terminal GND. The early turnoff circuit 16 includes a diode 10, a resistor 11, a pulse transformer 12, and an N-channel MOSFET 13. The pulse transformer 12 includes a primary coil 12A and a secondary coil 12B.
Next, an example of the operation of the DC-DC converter is described by using the operating waveforms in FIG. 7. At first, the power converting operation of the DC-DC DC converter is described. For example, the power switch driving signal having a pulse waveform as shown in part (c) of FIG. 7 is output from the power switch driving circuit of the control IC 4 to the gate of the power switch 3. Based on ON/OFF signals and OFF signals in the power switch driving signal, the power switch 3 performs an ON/OFF switching operation. A DC voltage input from an external DC input power supply 1 is converted by the switching of the power switch 3 into an AC voltage in the primary coil 2A of the main transformer 2, and is transmitted to the secondary coil 2F of the main transformer 2.
The synchronous rectifiers 5 and 6 on the side of the secondary coil 2B constitute a rectifying-and-smoothing circuit. In this rectifying-and-smoothing circuit, the AC voltage output from the secondary coil 2B of the main transformer 2 is rectified by the switching operations of the rectifying synchronous rectifier 5 and the commutating synchronous rectifier 6, which are described later, and is smoothed and converted into a DC voltage by the choke coil 7 and the capacitor 8. The DC voltage is supplied to an external load device 9 connected to the DC-DC converter. A signal in accordance with the output voltage is transmitted as a feedback signal to the signal input/output device 4 by a feedback loop (not shown). Based on the feedback signal, the power switch driving circuit of the control IC 4 controls the switching operation of the power switch 3, whereby the DC voltage supplied to the load device 9 is stabilized.
The rectifying synchronous rectifier 5 is turned on to be driven by a voltage generated in the ON period of the power switch 3 by the secondary coil 2B of the main transformer 2, and is turned off in the OFF period of the power switch 3. In other words, the rectifying synchronous rectifier 5 performs a switching operation with timing approximately synchronized with the turn-on and turnoff of the power switch 3.
Conversely, the commutating synchronous rectifier 6 is turned on by a reset pulse voltage in the secondary coil 2B of the main transformer 2 in the OFF period of the power switch 3, and is turned off in the OFF period of the power switch 3. The commutating synchronous rectifier 6 is an inversely driven synchronous rectifier whose ON/OFF switching operation is inverse with respect to that of the power switch 3.
In the example of the circuit in FIG. 6, before the power switch 3 is turned on, the commutating synchronous rectifier 6 can be turned off by the operations of the early turnoff circuit 16 and an ON-timing delay circuit (described later).
Next, an example of the operation of the early turnoff circuit 16 is described below.
As shown in part (c) of FIG. 7, when an ON signal for turning on the power switch 3 is output from the power switch driving circuit in the control circuit 4 at time t1, the ON signal applies a voltage to a series circuit of the primary coil 12A of the pulse transformer 12 and the gate (control terminal) of the power switch 3. Since the gate voltage of the power switch 3 is zero volts at time t1, a voltage output from the power switch driving circuit is entirely applied to the primary coil 12A of the pulse transformer 12. This causes the secondary coil 12B of the pulse transformer 12 to output a pulse signal as shown in part (e) of FIG. 7.
The pulse signal output from the secondary coil 12B is supplied to the gate (control terminal) of the N-channel MOSFET 13 and turns on the N-channel MOSFET 13. When the N-channel MOSFET 13 is turned on, as part (f) of FIG. 7 shows, at time t2, charge stored in the gate of the commutating synchronous rectifier 6 discharges to turn on the commutating synchronous rectifier 6.
Conversely, regarding the power switch 3, when the control IC 4 initiates outputting of an ON signal to the power switch 3 at time t1, the ON signal is supplied to the gate of the power switch 3 through the resistor 11 and the pulse transformer 12, and charge is added to the input capacitance of the power switch 3. Since the resistor 11 and the excitation inductance of the pulse transformer 12 operate as a delay factor, the power switch 3 has a gradual increase in gate voltage. When the gate voltage of the power switch 3 reaches a threshold value (time t3), the power switch 3 is turned on. A delay in the ON timing of the power switch 3 is set so that the ON timing of the power switch 3 is behind the turnoff of the commutating synchronous rectifier 6 by the early turnoff circuit 16, and the resistance of the conductive cover 11 and the excitation inductance of the pulse transformer 12 are set so that the delay is obtained. In other words, the resistor 11 and the pulse transformer 12 constitute an ON-timing delay circuit for delaying ON timing of the power switch 3.
After the power switch 3 is turned on (time t3), the voltage (drain-source voltage) across the power switch 3 starts to decrease, as shown in part (a) of FIG. 7. During the decease, the gate voltage of the power switch 3 is maintained to the threshold value by a mirror effect (see the period of time t3 to t4 in part (d) of FIG. 7). When the voltage across the power switch 3 reaches zero volts (time t4), the influence of the mirror effect turns off, thus restarting the gate voltage of the power switch 3. When the gate voltage of the power switch 3 reaches a power supply voltage of the control IC 4 (timing t5), the output of the pulse voltage from the pulse transformer 12 stops (see part (e) of FIG. 7).
When the output of the pulse voltage stops, a cyclic current flows in a path having a flowing order of the pulse transformer 12, the diode 10, and the pulse transformer 12, and a forward voltage drop in the diode 10 resets the exciting state of the pulse transformer 12. After an OFF signal for turning off the power switch 3 is output from the power switch driving switch in the control IC 4 (see part (c) of FIG. 7), the storage charge in the input capacitance of the power switch 3 discharges through the diode 10, thus turning off the power switch 3.
As described above, by setting a delay period from the time that the operations of the early turnoff circuit 16 and the ON-timing delay circuit cause the control IC 4 to output the ON signal to the power switch 3, to the time that the power switch 3 is turned on, and turning off the commutating synchronous rectifier 6 in the delay period, a short-circuiting current can be prevented from being generated by a delay in the turnoff of the commutating synchronous rectifier 6.
In addition to Japanese Unexamined Patent Application Publication No. 2000-262051, other examples of the related art are disclosed in Japanese Unexamined Patent Application Publication Nos. 10-174431, 11-206118, 2002-247848, 2002-247849, and 4-127869.
In the DC-DC converter in FIG. 6, the pulse signal output from the pulse transformer 12 has a fixed pulse width. In other words, the length of the delay operation period of the ON-timing delay circuit is fixed. Also, variations in component characteristics cause variations in turnoff timing of the commutating synchronous rectifier 6. When the variations in turnoff timing of the commutating synchronous rectifier 6 delay turnoff of the commutating synchronous rectifier 6, thus causing the power switch 3 to be turned on before the commutating synchronous rectifier 6 is turned off, a problem occurs in that a short-circuiting current is generated. Accordingly, in order to prevent the generation of the short-circuiting current even in a case in which turnoff timing of the commutating synchronous rectifier 6 is delayed, a delay in ON timing of the power switch 3 must be set to be large.
Nevertheless, when the delay in ON timing of the power switch 3 is excessive, a problem occurs in that an involved loss is generated. Specifically, in the circuit in FIG. 6, despite completion of turnoff of the commutating synchronous rectifier 6 at time t2, the delay operation of the ON-timing delay circuit continues to time t5. Accordingly, since charging to the input capacitance of the power switch 3 remains unchanged, there is a problem of an increase in switching loss caused by overlapping (see portion of timing t3 to t4 in parts (a) and (b) of FIG. 7) between the voltage across the power switch 3 and its current during the turn-on operation period of the power switch 3.